High strength and low shrinkage polyester yarn and process for its preparation

ABSTRACT

Disclosed is a high strength and low shrinkage polyester yarn, which has tenacity of 7.4 g/d or higher, elongation at break of 19 to 26%, shrinkage percentage of 2% or lower, and respective thermal-stress peaks of 3×10 −2  to 7.5×10 −2  g/d and 8.0×10 −2  to 10.5×10 −2  g/d at temperature ranges of 100 to 140° C. and 230 to 240° C., and process for its preparation. The high strength and low shrinkage polyester yarn is advantageous in that it has desirable resistance to external load and excellent dimensional stability, and is very useful as an industrial yarn with uniform shrinkage and excellent dimensional stability during a post-process for application to tarpaulin and truck covers.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates, in general, to a high strength andlow shrinkage polyester yarn and method of preparing the same and inparticular, to a high strength and low shrinkage polyester yarn, whichhas desirable resistance to external load and excellent dimensionalstability, and is useful as an industrial yarn with uniform shrinkageand excellent dimensional stability during a post-process forapplication to tarpaulins and truck covers, and a process for itspreparation.

[0003] 2. Description of the Prior Art

[0004] Having excellent physical and chemical properties, industrialuses for polyester yarn continuously increase. Particularly, highstrength polyester yarn is used as a base textile of coated textilessuch as tarpaulins and truck covers. However, because tarpaulins andtruck covers are prepared by coating the base textile with PVC at 180 to230° C., the high strength polyester yarn is nonuniformly shrunk due toits poor shrinkage property during a coating process, thus lackingdimensional stability. Furthermore, the high strength polyester yarnshrinks considerably during a post-process to degrade the quality of thecoated textile. Accordingly, there remains a need to develop a highstrength and low shrinkage polyester yarn.

[0005] One of processes of preparing a low shrinkage polyester yarn is awarp drawing process in which undrawn yarn (UDY) or partially orientedyarn (POY) wound after a spinning process is drawn, heat-treated, andrelaxed using separate devices. For example, Korean Patent PublicationNo. 1995-0000717 discloses the warp drawing process, in which undrawnyarn or partially oriented yarn is drawn and heat-treated to prepare apolyester yarn with tenacity of 8.0 g/d or higher, dry-heat shrinkingpercentage (190° C.) less than 2%, and elongation at break of 15 to 25%.However, the warp drawing process has disadvantages of high equipmentcost, reduced productivity, and increased production costs because ofthe separate drawing device.

[0006] An alternative process for preparing the low shrinkage polyesteryarn is a direct spinning drawing process disclosed in Japanese PatentLaid-open Publication No. Sho. 46-6459, in which spinning, drawing, andrelaxing steps are continuously performed. Korean Patent No. 0193940proposes a method of preparing a high elongation and low shrinkagepolyester yarn having a total draw ratio of 5.0 to 6.5, relaxation ratioof 10 to 15%, and fineness of 7 to 15 deniers by the direct spinningdrawing process. However, the direct spinning drawing process isdisadvantageous in that dry-heat shrinkage percentage is a high 3.3%when relaxation ratio is 12.7% even though residence time of thepolyester yarn on a roller is increased by slowing a spinning speed to600 m/min.

[0007] As described above, in the case of preparing the low shrinkagepolyester yarn by the direct spinning drawing process, if the total drawratio is increased to obtain a high strength yarn, a degree oforientation in the yarn is increased, thus undesirably increasing itsshrinkage ratio.

[0008] Additionally, if relaxation percentage is increased so as toreduce shrinkage, trembling of threads on a godet roller is undesirablyincreased to cause yarn breakage, thereby reducing workability.

[0009] Meanwhile, U.S. Pat. No. 5,277,858 suggests a method of preparinga low shrinkage polyester yarn having tenacity of 7.2 g/d or higher,shrinkage percentage less than 2.0% at 177° C., and shrinkage percentageless than 4.5% at 200° C. by the spinning-drawing continuous process.But this method is disadvantageous in that a separate heating device isneeded, which is used in a heating roller box. Furthermore, KoreanPatent Laid-open Publication No. 1998-028329 discloses a method ofpreparing a low shrinkage yarn, in which a heating and a cooling device,and a steam supplying device are additionally set between godet rollers.However, a large space as well as additional devices such as the heatingand cooling device and the steam supplying device are needed so as tocommercialize this method, thus reducing economic efficiency.

SUMMARY OF THE INVENTION

[0010] Therefore, the present invention has been made keeping in mindthe above disadvantages occurring in the prior art, and an object of thepresent invention is to provide a high strength and low shrinkagepolyester yarn, which has desirable resistance to external load and isuseful as an industrial yarn with uniform shrinkage and excellentdimensional stability during a post-process for application totarpaulins and truck covers.

[0011] It is another object of the present invention to provide a methodof preparing the high strength and low shrinkage polyester yarn.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

[0013]FIG. 1 is a graph showing heat stress as a function of temperaturefor a high strength and low shrinkage polyester yarn by example 1 of thepresent invention, and for two conventional types of polyester yarns;

[0014]FIG. 2 is a graph showing load as a function of time for the highstrength and low shrinkage polyester yarn by example 1 of the presentinvention, which illustrates a shrinkage behavior of the high strengthand low shrinkage polyester yarn; and

[0015]FIG. 3 is a flow diagram of a procedure of preparing the highstrength and low shrinkage polyester yarn by the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention provides a high strength and low shrinkagepolyester yarn, which has tenacity of 7.4 g/d or higher, elongation atbreak of 19 to 26%, shrinkage percentage of 2% or lower, and respectivethermal-stress peaks of 3×10⁻² to 7.5×10⁻² g/d and 8.0×10⁻² to 10.5×10⁻²g/d at temperature ranges of 100 to 140° C. and 230 to 240° C.

[0017] The high strength and low shrinkage polyester yarn ischaracterized in that a ratio of a thermal-stress peak at a temperaturerange of 230 to 240° C. to a thermal-stress peak at a temperature rangeof 100 to 140° C. is 1.3 to 3.0, and a shrinkage force of the polyesteryarn within the first 5 sec after the start of shrinkage is 4.5×10⁻² to6.5×10⁻² cN/d and the shrinkage force of the polyester yarn thereafteris 1.5×10⁻² to 3.5×10⁻² cN/d.

[0018] Furthermore, the present invention provides a method of preparinga high strength and low shrinkage polyester yarn by the a directspinning drawing process, comprising (a) spinning a melted polyesterpolymer at a speed of 383 to 490 m/min, (b) drawing a spun polyesteryarn in a total draw ratio of 5 to 6.4, and (c) relaxing a drawnpolyester yarn at 230 to 250° C. by a godet roller in relaxation ratioof 9 to 13%.

[0019] The relaxation is performed through a first relaxing step and asecond relaxation step, and a relaxation distribution ratio of the firstrelaxation step to the second relaxation step is 9:1 to 1:9.

[0020] A more detailed description of the high strength and lowshrinkage polyester yarn and the method of preparing the same will begiven, below.

[0021] The high strength and low shrinkage polyester yarn of the presentinvention is prepared by the a direct spinning drawing process.

[0022] In detail, a polyester polymer is melted and then spun at a speedof 383 to 490 m/min. In consideration of spinning workability and lowshrinkage property, it is preferable to use the polyester polymer withan intrinsic viscosity of 0.74 to 0.95. After 0.4% polyester solution isprepared using a mixed solvent, in which phenol is mixed with1,1,2,2-tetrachloroethane in a mixing ratio of 6:4, to measure a passingtime ratio of the polyester solution to the mixed solvent through astandard capillary by use of an Auto Visc II viscometer manufactured byCanon Co., the intrinsic viscosity (IV) of the polyester polymer iscalculated according to a following Bill-Meyer Equation:${IV} = {\frac{{RV} - 1}{4C} + \frac{3{\ln ({RV})}}{4C}}$

[0023] (wherein, C is a concentration of the polyester polymer in thepolyester solution (g/100 ml))

[0024] An undrawn polyester yarn is then drawn at a total draw ratio of5 to 6.4. When the draw ratio is less than 5, orientation of the yarn ispoor, thus not obtaining desired strength. On the other hand, when thedraw ratio is more than 6.4, the yarn is over-drawn to cause a singleyarn-breakage, thereby reducing workability or causing a full yarnbreakage. The drawn polyester yarn is relaxed in relaxation ratio of 9to 13% at 230 to 250° C. At this time, the polyester yarn isheat-treated by a godet roller. When the relaxation ratio is less than9%, it is difficult to obtain the desirable low shrinkage polyesteryarn, and when the relaxation ratio is more than 13%, trembling ofthreads on the godet roller is undesirably increased, thereby reducingworkability.

[0025] A relaxation process may be performed through a single step, butit is preferable that the relaxation process is performed through twosteps, i.e. a first relaxation step and a second relaxation step. Atthis time, a relaxation distribution ratio of the first relaxation stepto the second relaxation step is preferably 9:1 to 1:9. When thepolyester yarn is relaxed through two steps, trembling of threads on thegodet roller is reduced and heat-treatment efficiency is improved due tosufficient residence time of the polyester yarn on the godet roller, soan actual relaxation ratio reaches an available relaxation ratio toimprove shrinkage property of the polyester yarn. When a temperature ofthe godet roller is less than 230° C., relaxation efficiency is reducedbecause of insufficient heat-treatment efficiency, thus the polyesteryarn is poor in terms of low shrinkage property. On the other hand, whenthe temperature is more than 250° C., tenacity of the polyester yarn isreduced due to thermal decomposition of the yarn. The relaxed polyesteryarn may be wound at a speed of 2000 m/min or faster.

[0026] According to the method of preparing the high strength and lowshrinkage polyester yarn of the present invention, a separate coolingdevice and heating device are not necessary because the roller emitsheat, and the polyester yarn with excellent physical propertiesincluding tenacity of 7.4 g/d or higher, elongation at break of 19 to26%, and shrinkage percentage of 2% or lower is obtained.Conventionally, these desirable physical property values could beobtained from a warp drawing process, that is to say, a spinning-drawingtwo step process which has higher heat-treatment efficiency than aspinning-drawing continuous process.

[0027] The high strength and low shrinkage polyester yarn of the presentinvention has higher crystallinity and lower amorphous orientation thana conventional high strength and low shrinkage yarn prepared by thecontinuous process, in a view of microstructure. Furthermore, thepolyester yarn by the present invention is characterized in that it haslower crystallinity, less tie molecules, and more amorphous regions withlow orientation than the conventional high strength and low shrinkageyarn prepared by the warp drawing process. The reason for this is thatcrystallization of an amorphous regions with high orientation is inducedunder conditions of desirable spinning speed, total draw ratio,relaxation ratio, and relaxation temperature, so amorphous regionsdwindle away and, if existing, the amorphous region has a low degree oforientation due to high relaxation efficiency. That is to say, an amountand a degree of orientation of oriented amorphous regions which areshrunk to a high randomness state are reduced, thereby improving the lowshrinkage property of the polyester yarn.

[0028] Meanwhile, the high strength and low shrinkage polyester yarn bythe present invention has several unique thermal characteristics asfollows. That is to say, it has two thermal-stress peaks of 3×10⁻² to7.5×10⁻² g/d and 8.0×10⁻² to 10.5×10⁻² g/d at temperature ranges of 100to 140° C. and 230 to 240° C., respectively. These are thermalcharacteristics different from high strength and low shrinkage polyesteryarns by the conventional direct spinning drawing process and warpdrawing process.

[0029]FIG. 1 is a graph showing thermal-stress as a function oftemperature for the high strength and low shrinkage polyester yarnaccording to example 1 of the present invention, and for twoconventional types of polyester yarns.

[0030] With reference to FIG. 1, a high strength and low shrinkagepolyester yarn (195ST) manufactured by ACODiS Co., LTD and another highstrength and low shrinkage polyester yarn (HELS2) manufactured by KOLONINDUSTRIES, Inc. by the warp drawing process each have a singlethermal-stress peak at a high temperature region. On the other hand, thehigh strength and low shrinkage polyester yarn (HS) of the presentinvention has two thermal-stress peaks, one at a low temperature and theother at a high temperature region. The reason for this ismicrostructural difference within the fiber resulting from differentmanufacturing process and thermal history. The high strength and lowshrinkage polyester yarn by the warp drawing process has lowthermal-stress at a low temperature because of low amorphousorientation, but the high strength and low shrinkage polyester yarn bythe direct spinning drawing process has high thermal-stress at a lowtemperature because of much amorphous regions and a higher amorphousorientation than the fiber by the warp drawing process.

[0031] Meanwhile, the high strength and low shrinkage polyester yarn bythe present invention is characterized in that a ratio of athermal-stress peak at a temperature range of 230 to 240° C. to athermal-stress peak at a temperature range of 100 to 140° C. is 1.3 to3.0, and a shrinkage force of the polyester yarn within 5 sec after theonset of polyester yarn shrinkage is 4.5×10⁻² to 6.5×10⁻² cN/d and theshrinkage force of the polyester yarn after 5 sec is 1.5×10⁻² to3.5×10⁻² cN/d, thereby securing excellent tenacity, shrinkagepercentage, and elongation at break (refer to FIG. 2).

[0032] A better understanding of the present invention may be obtainedin light of the following examples which are set forth to illustrate,but are not to be construed to limit the present invention.

EXAMPLE 1

[0033] Polyester chips with intrinsic viscosities of 0.84 produced by asolid state polymerization process were melted, spun through a spinningnozzle at a speed of 430 m/min, and cooled. A spun undrawn yarn 10 waspassed through a device 20 for supplying oil and then drawn between afirst roller GR1 and a fourth roller GR4 for a undrawn yarn. At thistime, a speed of the fourth roller GR4 is adjusted to 2450 m/min so thata total draw ratio io is (refer to FIG. 3). Temperatures of the fourthroller GR4 and a fifth roller GR5 were all controlled to 240° C. toperform first and second heat-setting for the polyester yarn, and afirst relaxation ratio between the fourth roller GR4 and fifth rollerGR5 was controlled to 0.7% and second relaxation ratio between the fifthroller GR5 and a sixth roller GR6 was controlled to 3% so that totalrelaxation ratio was 10%. The second to fifth rollers GR2 to GR5 arepositioned in a box 30 to keep insulated. A relaxed yarn was wound by awinding device 40 and the resulting polyester yarn was 1000 deniers infineness.

EXAMPLES 2 TO 9 AND COMPARATIVE EXAMPLES 1 TO 7

[0034] The procedure of example 1 was repeated except that total drawratio, temperatures of GR4 and GR5, relaxation ratio, and relaxationdistribution ratio described in Table 2 were different from those ofexample 1. Physical properties of polyester yarns by examples 1 to 9 andcomparative examples 1 to 7 were measured and the results are describedin Tables 1 and 2.

[0035] In order to measure strength and elongation at break of a greyyarn, a sample of 250 mm was twisted in 80 turns/m a

dected to a tensile load with a test speed of 300 mm/min according toASTM D885. Tenacity of the grey yarn was determined by dividing themeasured strength of the grey yarn by weight of the grey yarn with alength of 9000 m.

[0036] Shrinkage percentage of the grey yarn was determined by measuringlength difference of a sample before and after the sample was left at190° C. for 15 min while being applied by a load of 0.01 g/d.Additionally, a shrinking force of the grey yarn was determined bymeasuring shrinkage of the sample for 1 min after the sample was nippedby being applied with a pretension of 0.01 g/d at 200° C.

[0037] A Kanebo Thermal Stress Tester (type KE-1) was used to observecontinuous thermal-stress behavior, whereby the sample was formed into aloop, and drawn between two hooks to apply a pre-tension of 0.05 g/d(100 g in case of 1000 d, or 50 g in case of 500 d), and heated from aroom temperature to 300° C. at a rate of 200° C./min.

[0038] Workability was determined by measuring the generation of fluffusing a fluff counter manufactured by Daiko Co. of Japan, positionedbeforefore a .Ung machine. Based on 10 kg of the wound sample, if ameasured value is 3 or lower, workability is considered to be excellent,and if the value is 4 or higher, workability is considered to be normal.

[0039] The draw ratio, relaxation ratio, and relaxation distributionratio are defined as follows:

Draw ratio=rotation speed of GR 4/rotation speed of GR 1

Relaxation ratio=first relaxation ratio+second relaxation ratio.

First relaxation ratio={(rotation speed of GR 4−rotation speed of GR5)/rotating speed of GR 4}×100

Second relaxation ratio={(rotation speed of GR 5−rotation speed of GR6)/rotation speed of GR 5}×100

Relaxation Distribution ratio=first relaxation ratio/second relaxationratio TABLE 1 Examples 1 2 3 4 5 6 7 8 9 ¹Total D.R. 5.7 6.0 6.2 6.4 5.56.2 6.4 6.0 6.2 ²T. of GR4/5 240/240 250/250 250/250 240/180 240/240230/230 250/250 244/190 244/244 ³Relaxation 10 12 12 13 9 13 13 11 11.5⁴Distribution 7:3 9:1 6:4 10:0 8:2 1:9 9:1 10:0 7:3 ⁵Tenacity 7.8 8.18.4 8.5 7.4 8.2 8.4 7.5 8.2 ⁶Break. 24.5 23.2 23.8 22.5 25.4 24.2 19.723.2 23.5 ⁷Shrinkage 1.4 1.5 1.3 1.7 1.5 1.7 1.3 1.7 1.1 ⁸Max. shr. 5.85.7 4.8 6.1 5.9 6.2 4.7 6.3 4.5 ⁹Final shr. 3.1 3.1 2.0 3.3 3.1 3.2 1.93.3 1.8 ¹⁰Peak 110/231 110/234 105/232 130/230 140/235 102/236 118/231123/233 128/235 ¹¹Thermal- 4.7/9.2 5.3/9.1 4.5/8.8  7.2/10.1 5.2/9.07.4/9.6 4.3/9.0 7.4/10.2 3.1/9.5 stress Workability Excel. Excel. Excel.Excel. Excel. Excel. Excel. Excel. Excel.

[0040] TABLE 2 Comp. Examp. 1 2 3 4 5 6 7 ¹Total D.R. 4.0 7.0 6.0 5.76.0 5.9 6.2 ²T. of GR4/5 230/230 230/230 240/180 240/190 255/190 220/220220/220 ³Relaxation 9 9 14 8 13 9 10 ⁴Distribution 7:3 8:2 10:0 10:010:0 8:2 2:8 ⁵Tenacity 6.5 — 7.2 7.5 7.3 7.5 7.9 ⁶Break. 26.7 — 23.624.7 23.8 23.1 22.5 ⁷Shrinkage 1.4 — 1.8 2.2 1.8 5.3 5.5 ⁸Max. shr. 6.5— 6.5 7.1 7.2 7.4 7.8 ⁹Final shr. 5.0 — 5.1 6.0 6.2 4.2 4.5 ¹⁰Peak125/232 — 112/232 128/235 105/230 123/232 124/237 ¹¹Thermal- 4.5/9.4 —6.5/9.8  8.2/10.3 6.2/9.5 10.3/11.2 10.5/11.5 stress Workability Excel.Breakage Normal Excel. Normal Excel. Excel.

[0041] From the results of Table 1, it can be seen that the polyesteryarn of the present invention (examples 1 to 9) having thermal-stresspeaks of 3×10⁻² to 7.5×10⁻² g/d and 8.0×10⁻² to 10.5×10⁻² g/d attemperature ranges of 100 to 140° C. and 230 to 240° C., respectively,has excellent tenacity and shrinkage property, thus desirably reducingyarn breakage, monofilament yarn breakage, and tar occurrence on theroller, thereby improving workability. The reason for this is that atotal draw ratio, a relaxation temperature, a relaxation ratio, and arelaxation distribution ratio are desirably controlled.

[0042] Meanwhile, in the case of comparative example 1, workability isexcellent but the total draw ratio is too low to secure desirablephysical properties and in the case of comparative example 2, the totaldraw ratio is too high, thus causing yarn breakage. The yarn accordingto comparative example 3 has excessively high relaxation ratio, so it isdifficult to secure desirable workability due to severe trembling ofthreads on the godet roller.

[0043] Furthermore, the yarn by comparative example 4 does not obtainshrinkage of 2% or lower when the relaxation ratio is 8%. Additionally,the yarn by comparative example 5 has low shrinkage property at therelaxation ratio of 13%, but has too low yarn tenacity because of a hightemperature of the godet roller and has disadvantages of yarn breakagedue to contamination of the godet roller. In the case of comparativeexamples 6 and 7, the shrinkage percentage is undesirably 2% or higherbecause a temperature of the relaxation region is not sufficiently high.

[0044] As described above, a high strength and low shrinkage polyesteryarn by the present invention is advantageous in that it has excellentphysical properties such as tenacity of 7.4 g/d or higher, elongation atbreak of 19 to 26%, and shrinkage percentage of 2% or lower, thussecuring sufficient resistance to external load and excellentdimensional stability with uniform shrinkage during a post-process.Therefore, the polyester yarn according to the present invention is veryuseful as an industrial yarn such as tarpaulin and truck covers.

[0045] The present invention has been described in an illustrativemanner, and it is to be understood that the terminology used is intendedto be in the nature of description rather than of limitation. Manymodifications and variations of the present invention are possible inlight of the above teachings. Therefore, it is to be understood thatwithin the scope of the appended claims, the invention may be practicedotherwise than as specifically described.

What is claimed is:
 1. A high strength and low shrinkage polyester yarn,which has tenacity of 7.4 g/d or higher, elongation at break of 19 to26%, shrinkage percentage of 2% or lower, and respective thermal-stresspeaks of 3×10⁻² to 7.5×10⁻² g/d and 8.0×10⁻² to 10.5×10⁻² g/d attemperature ranges of 100 to 140° C. and 230 to 240° C.
 2. The highstrength and low shrinkage polyester yarn as set forth in claim 1,wherein a ratio of a yarn's thermal-stress peak at a temperature rangeof 230 to 240° C. to a thermal-stress peak at a temperature range of 100to 140° C. is 1.3 to 3.0.
 3. The high strength and low shrinkagepolyester yarn as set forth in claim 1, wherein a shrinkage force of thepolyester yarn within a first 5 sec after the start of shrinkage is4.5×10⁻² to 6.5×10⁻² cN/d, and the shrinkage force of the polyester yarnthereafter is 1.5×10⁻² to 3.5×10⁻² cN/d.
 4. The process for preparing ahigh strength and low shrinkage polyester yarn by the direct spinningdrawing process, comprising: (a) spinning a melted polyester polymer ata speed of 383 to 490 m/min; (b) drawing a spun polyester yarn in atotal draw ratio of 5 to 6.4; and (c) relaxing a drawn polyester yarn at230 to 250° C. by a godet roller with a relaxation ratio of 9 to 13%. 5.The process according to claim 4, wherein the relaxation is performedthrough a first relaxation step and a second relaxation step, and arelaxation distribution ratio of the first relaxation step to the secondrelaxation step is 9:1 to 1:9.